1. Field of the Invention
This invention relates to a coating apparatus, more particularly, a coating apparatus for coating pharmaceutical or food tablets or the like with a layer of other medical composition or sugar etc., which apparatus includes a rotatable coating pan, the side wall of which has a ventilating area or areas for feeding dry air as a ventilating and drying mechanism.
2. Description of the Prior Art
When, in a coating apparatus, a coating material, such as a film liquid or a sugar solution, is sprayed on the surface of a material to be coated, such as tablets or a granular raw material, and thereafter, the coating material is dried, there has heretofore been adopted a method wherein the tablets as being the material to be coated are tumbled in a coating pan by the rotation of the coating pan, while heated air is blown onto the surface of a bed of the tablets to dry the tablets. However, according to the conventional method of the type described, although it is possible to relatively sufficiently dry only the tablets in the surface zone of the bed, the tablets in the inside and lower zones of the bed are insufficiently dried because the heated gas cannot reach such zones. Therefore, it takes long time to dry all of the tablets in the coating pan, and it is difficult to dry the tablets uniformly.
For eliminating these drawbacks, the present applicant has proposed a coating apparatus shown in FIG. 1 and 2, which is disclosed in the Japanese Patent Publication No. 38713/1975. This coating apparatus is provided with a ventilating and drying mechanism comprised of perforated areas 2 for ventilating air, arranged axially in a plurality of circumferential positions of whole circumference of a rotatable coating pan or rotatable drum 1, an air supply conduit 3, an exhaust or suction duct 4, and exhaust conduit 5. In this coating apparatus, the heated air blown into the coating pan 1 from the air supply conduit 3 is blown onto tablets 6 tumbled in the coating pan 1, and after passing through the bed of the tablets 6, the heated air is exhausted from the exhaust conduit 5 via the perforated area 2 and the exhaust duct 4. Therefore, this coating apparatus is used as an excellent coating apparatus in practice, because, in this coating apparatus, the heated gas can contact uniformly with the tablets 6 not only in the surface zone but also in the inside and lower zones of the bed thereof while passing through the bed of the tablets, the heat efficiency is very high, and it is possible to obtain homogeneous product dried with overall uniformity and with high productivity.
Further, as the result of the high heat efficiency provided by this coating apparatus, it is possible to perform coating operation efficiently and in low cost, using an aqueous coating solution wherein coating material is dissolved or dispersed in the water (in this connection, refer to the Japanese Patent Publication No. 5491/1980).
Moreover, in FIGS. 1 and 2, the reference numeral 7 is a rotary shaft for rotating the coating pan 1, 8 is a motor for driving the rotary shaft 7 via transmission means 9 such as a belt or a chain, 10 is a cylindrical portion which forms a charging opening through which tablets 6 to be coated are charged into the coating pan 1, and 11 is a cover for the cylindrical portion 10. These cylindrical portion 10, cover 11 and air supply conduit 3 and exhaust conduit 5 do not rotate. The connecting portions between these non-rotating members and rotating members of the coating pan 1 etc. are sealed by labyrinth type sealing means etc.
Now, in such a coating apparatus as above, there are provided a plurality of ventilating through apertures having circular, rectangular or elliptic shapes in the perforated area 2 to increase the numerical aperture of the perforated area 2 in order to reduce the resistance to the ventilation through the perforated area 2. But, there is a possibility of causing such a phenomenon that, as a large quantity of the material to be coated is processed in the coating pan, the tablets or other granular materials being coated are clogged in the ventilating apertures to block the apertures. When the ventilating apertures are blocked, the smooth flow of the heated gas is prevented to produce non-uniform ventilation, and the pressure loss of the ventilation system becomes increased or changed to make the coating operation instable. Further, it takes longer time for drying because of the reduction of the heat efficiency.
Alternatively, there is proposed other coating system, in which a powdery raw material is used in place of a portion of coating solution to reduce the energy for drying by using the above-described coating apparatus. However, in this coating system, because the powdery material is leaked through the ventilating aperture and the leaked powdery material is let out of the system through the exhaust duct and conduit, not only a product having non-uniform components is produced, but also loss of expensive powdery raw material and environmental pollution occur.
Such problems arise not only in a conical shaped coating apparatus, but also in other cylindrical coating apparatus disclosed in the Japanese Utility Model Publication No. 21466/1981, or an onion or a pear shaped coating apparatus.
Then, to eliminate the above-described drawbacks, the present applicant has proposed a construction having a baffle or scoop plate covering the ventilating areas within the coating pan (Japanese Patent Application No. 174273/1982). This construction can offer such outstanding advantages that it can prevent the leak of powdery material through ventilating areas and blocking of the ventilating areas, increase the drying efficiency, produce homogeneously coated products and so on.
Now, as the apparatus of the type described, necessity has been voiced for achieving combined effects of smoothing the material to be coated, controlling damages thereof and occurrence of noises therefrom, increasing the effect of mixture, further, raising the rate of charge and so on, in addition to the prevention of the leak of powdery material.